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| Dissertation / PhD Thesis/Book | PreJuSER-14775 |
2010
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
Jülich
ISBN: 978-3-89336-746-7
Please use a persistent id in citations: http://hdl.handle.net/2128/4525
Abstract: The thesis presents thermo-mechanical properties of La$_{0.58}$Sr$_{0.4}$Co$_{0.2}$Fe$_{0.8}$O$_{3-\delta}$ (LSCF) and Ba$_{0.5}$Sr$_{0.5}$Co$_{0.8}$Fe$_{0.2}$O$_{3-\delta}$ (BSCF) perovskite materials, which are considered as oxygen transport membranes (OTM) for gas separation units. Ring-on-ring bending test with disk-shaped samples and depth-sensitive micro-indentation have been used as macroscopic and microscopic tests, respectively. In addition, the thermo-mechanical properties of a third OTM candidate material La$_{2}$NiO$_{4+\delta}$(LNO) were investigated. The results of the thermo-mechanical measurements with the BSCF revealed an anomaly between 200 °C and 400 °C. In particular, the temperature dependence of Young’s modulus shows a minimum at ~ 200 °C. Fracture stress and toughness exhibit a qualitatively similar behavior with a minimum between 200 °C and 400 °C, before recovering between 500 °C and 800 °C. X-ray diffraction analyses verified that BSCF remains cubic in the relevant temperature range. Hence the anomalies were assumed to be related to the transition of Co$^{3+}$ spin states reported for other Co-containing perovskites. This assumption could be experimentally confirmed by magnetic susceptibility measurements. The fracture surfaces of the specimens are not affected by the mechanical anomalies at intermediate temperatures, since only a transgranular fracture mode has been observed. Complementary to the mechanical characterization of BSCF, also the temperature dependency of fracture stress and elastic behavior of LSCF have been determined. Phase compositions of LSCF have been studied by in-situ high temperature XRD. Changes in phase composition with temperature are observed. At ambient temperature the LSCF perovskite material comprises two phases: rhombohedral and cubic symmetry. The ratio of the two phases depends on both cooling rate and atmosphere. The transition of rhombohedral to cubic occurs between 700 °C and 800 °C in air. The transition appears to be associated with a change of heat capacity. The [...]
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